In recent years, a keyless offset printing machine, which excludes an apparatus (ink adjusting buttons) for adjusting an ink amount in order to simplify a printing machine, has been increasingly used. This keyless off-set printing machine has main purposes of simplifying a structure of a printing machine, decreasing a manufacturing cost, and allowing an unskilled operator to operate the machine. That is, conventional printing machines have a large number of ink adjusting buttons for adjusting an ink amount in the widthwise direction of an object to be printed. An ink amount required for printing is adjusting by periodically monitoring the object to be printed. The keyless offset printing machine will be described below with reference to FIG. 1.
Referring to FIG. 1, reference numerals 1 denote ink fountains which contain ink 2. Ink fountain rollers 4 are located above the ink fountains 1 to draw up the ink 2 from the ink fountains 1 and form ink films 3 on their surfaces. Metering rollers 5 are located above the ink fountain rollers 4 to receive the ink from the ink fountain rollers 4 and adjust metering. As the metering rollers 5, a roller called an anilox roller manufactured by forming a large number of independent small recesses (cells) for holding ink on the surface of a core metal (not shown) is generally used. A large number of independent pyramidal recesses 2a are formed on the surface of the anilox roller as shown in FIGS. 2(A) and 2(B), or a large number of pyramidal trapezoidal recesses 3a are formed thereon as shown in FIGS. 3(A) and 3(B). Doctor blades 6 made of steel (e.g., Sweden steel) are located in contact with the metering rollers 5 to scrape off excessive ink from the surfaces of the metering rollers 5. Rubber forme rollers 8 are located above the metering rollers 5 to supply the ink from the metering rollers 5 to forme cylinders. Rubber blanket cylinders 12 are located adjacent to the forme rollers 8 via forme cylinders 7 to transfer predetermined printing contents onto an object to be printed 13 such as paper. Dampening water 10 of dampening arrangements 9 is supplied to non-image areas of formes of the forme cylinders 7 via soaking rollers 11.
The surface layer of the metering roller 5 of the above keyless offset printing machine is molded as follows. That is, the surface of a steel roll (mother) having a large number of projections is urged against the surface of a core metal consisting of, e.g., iron, thereby forming the recesses 2a or 3a shown in FIG. 2 or 3, respectively. Chromium plating is then performed on the surface of the core metal. This chromium plating is performed to protect the surface of the core metal from abrasion caused by the doctor blade. The number of recesses 2a or 3a formed on the surface of the metering roller 5 serving as the anilox roller is represented by the number of recesses 2a or 3a arranged in a width of an inch. For example, "300 lines/inch" means that 300 recesses 2a or 3a are arranged in a width of an inch. The depth of each recess 2a or 3a is normally about 25 .mu.m. In place of chromium plating, a ceramic such as tungsten carbite is sometimes flame-sprayed on the surface of the core metal.
Unlike the molding method of the recesses 2a or 3a using the mother, another molding method may be performed such that a ceramic is flame-sprayed on the surface of the core metal and then a laser beam is radiated thereon to form the recesses 2a or 3a on the surface of the core metal.
A roller for serving as the anilox roller of the keyless offset printing machine must satisfy the following conditions.
1 Ink density is not reduced by dampening water, and roller stripping (a phenomenon in which ink cannot be spread on the roller due to dampening water) does not occur.
2 The roller itself is not easily abraded, the shape of the recess (cell) is not changed, ink holding and supply are stably performed, and this state is maintained for about a year.
3 The roller itself can be easily repaired.
4 The shape or size of the recesses (cells) has no variation in a single roller or between rollers.
5 An ink holding amount, i.e., ink density can be varied.
2 The roller does not abrade a doctor blade.
The conventional roller used as the anilox roller, however, does not satisfy the above conditions and has the following drawbacks.
1 Since copper, ceramic, or the like has no sufficient affinity with ink and does not have an ink suction property, density reduction or roller stripping often occurs due to ink holding property degradation caused by dampening water. When an alcohol is added in the dampening water, this phenomenon becomes more significant.
2 Since the anilox roller made of a conventional material is easily abraded, the recesses (cells) are abraded, and ink density is reduced. Therefore, the conventional anilox roller must be replaced about every month.
3 When a hard object hits the roller, the roller is scratched or cracked. Therefore, since the roller cannot be repaired, it must be remade.
4 Since a force is physically applied to the anilox roller molded by urging a mother against a core metal, the shape of the recesses has large variations in a single roller and between rollers. For this reason, ink density variation occurs in printed objects.
5 Since the pattern of the recesses is predetermined, ink density cannot be increased or decreased.
6 Since a shoulder portion of the recess is sharp, a doctor blade is rapidly abraded by an anilox roller consisting of a super hard material and must be replaced with a new one after the use of about one week.
7 In solid printed portion (a portion at which the entire printing surface is covered with ink), the pattern of the independent recesses is reproduced on a printed object.
8 In order to mold the recesses by flame spraying or laser beam sculpture, an enormous installation cost is required.
9 When color ink having a high ink tack is used, required ink density cannot be obtained because ink filling and holding properties are poor.